The field of art to which the claimed invention pertains is adsorptive separation. More specifically, the invention relates to a process for separating hydroxy paraffinic dicarboxylic acids from a feed mixture comprising hydroxy paraffinic dicarboxylic acids and olefinic dicarboxylic acids which process employs a particular non-zeolitic, hydrophobic, crystalline silica molecular sieve adsorbent material to selectively reject and recover the hydroxy paraffinic dicarboxylic acids from the feed. In addition the process of the invention may preferably employ a means to enhance the selective adsorption of the extract components in the adsorption zone consisting of the introduction of an unadsorbed material into the adsorption zone of the process, such means being hereinbelow referred to as a "pre-pulse technique".
One commercially important hydroxy paraffinic dicarboxylic acid is malic acid. Malic acid occurs naturally in certain fruits, is the predominant acid in apples and is recoverable therefrom. Malic acid may also be synthesized by the hydrolysis of maleic acid. Unfortunately, the hydrolyzate not only contains malic acid, but also contains minority components comprising olefinic dicarboxylic acids such as unreacted maleic acid and a chemical intermediate, fumaric acid.
Substantial use of malic acid is made in the food industry as an acidulant, as an alternative to other materials such as citric acid. Malic acid is slightly less soluble than citric acid, when used as an acidulant, but it exhibits a stronger apparent acid taste which peaks more slowly thereby more effectively masking aftertastes. Malic acid is most commonly used in soft drinks, dry mix beverages, puddings, jellies and fruit fillings. Additionally, malic acid's melting point (129.degree. Celsius) is lower than that of citric acid and, as a result, is more preferably utilized in the production of hard candies. It is known that at temperatures above about 100.degree. to 150.degree. Celsius, malic acid slowly dehydrates and reverts to fumaric acid. Although fumaric acid is likewise useable as an acidulant, its aqueous solubility is poorer than that of malic acid. Correspondingly, maleic acid, although used in the food industry as a preservative for fats and oils, is moderately toxic and is a strong irritant to tissue. Therefore, the removal of maleic acid and fumaric acids from the hydrolysis reaction product mixture produces a superior malic acid product.
It is therefore an object of the present invention to provide a process for the separation of commercially important hydroxy paraffinic dicarboxylic acids, such as malic acid, from a commercially available feed mixture comprising hydroxy paraffinic and olefinic dicarboxylic acids. In the preferred embodiment, the feed mixture is derived from the hydrolysis reaction of maleic acid and comprises a mixture of fumaric, malic and maleic acids, the adsorbent comprises silicalite, the desorbent comprises aqueous acetone and the process is carried out in a simulated countercurrent moving bed system using a pre-pulse technique.